Conductivity measurement probe

ABSTRACT

A conductivity measurement probe formed of an electrically nonconducting rigid body which carries a cylindrical flow passage defining a flux channel to polarization current. The body has inlet and outlet means for passing aqueous liquid or other electrolytes through the flow passage. Four elongated metallic electrodes are positioned in alignment within the flow passage and in electric isolation from one another. Preferably, the outer electrodes serve as references for observing induced polarizing potential and an inner pair of the electrodes which serve auxiliary functions for providing current flow in the flow passage; and, the reference electrodes reside within the limits of the flux channel containing the polarization current. The reference and auxiliary electrodes are preferably spaced uniformly along the flow passage. Electrically conductive means extend from the electrodes within the body. These conductive means are insulated from one another and the body to form circuits through the electrodes during conductivity measurements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the measurement of electricalconductivity of a liquid, and particularly, it concerns an improvedconductivity probe for use with electrical conductivity measuringinstruments.

2. Description of the Prior Art

The determination of the conductivity of an electrolyte such as anaqueous liquid has been an elusive task for over one hundred years. Amultitude of techniques and apparatuses were tested before relativelysatisfactory systems were found for this purpose. The greatest error inprior attempts to measure electrolytic conductivity were caused by thepolarization of electrodes which were employed for potential sensing andcurrent source purposes. Attempts to avoid the problem with thepolarization of electrodes resulted in two separate electrical systemsbeing developed for measurement of conductivity.

The polarization of electrodes can be eliminated by using alternatingcurrent of relatively high frequency and stability combined with veryspecial electrodes coated with platinum black. These electrodes expose alarge surface to an electrolyte so as to reduce the surface density ofions being deposited and thereby reducing the polarization effects.

The use of d.c. measurement system for determining conductivity hasseveral advantages, the principal advantage being in avoiding aprecisely settable and stable high frequency oscillator for generatingthe alternating currents of the preceding a.c. measurement system. Manyattempts were made to employ d.c. currents for the measurement ofconductivity of aqueous liquids and other electrolytes. Nearly onehundred years ago, a method was developed in which the polarizationeffects of electrodes could be reduced to manageable proportions forproducing reliable conductivity measurements. In such a method, aconstant current is passed through the electrolyte and the drop inpotential between two points in the system is measured by secondaryelectrodes connected to an electrometer.

The use of such a plurality of electrodes implements a novel techniquewhere current flow between a first set of electrodes creates apolarization potential between an additional set of potential monitoringelectrodes. In such arrangement, the change in current can be correlatedto the change of polarization potential at the reference electrodes.Thus, such measurements can be correlated directly to the electromotiveforce which opposes the flow of current through the electrolyte. Theproblem of polarization at the electrodes can be overcome by applying asmall finite current flow and measuring the induced polarizationpotential change. Pursuant to Ohm's Law, the smallest polarizationpotential change is induced by a corresponding current flow. Theoriginal polarization potential at the measurement electrodes iseliminated in a linear system. Now, the measurement system follows Ohm'sLaw, the polarization potential about the measurement electrodes and thecurrent flow inducing same are directly proportional and simultaneouslyapproach zero magnitudes.

In many of the known direct current measurement systems for determiningconductivity of electrolytes, the measurement is made in what is termedan "open" cell. The electrodes are immersed within a liquid, but thesystem is exposed to atmospheric and other external forces, which forcesundesirably effect the current flow between the electrodes. Thus,movement of the electrodes relative to the container, the placement intothe solution of some foreign material such as an air bubble, andstirring or like mechanical displacement, severely affected the accuracyof these d.c. measurements with electrodes in an open cell.

Complicated conductivity probe design may be employed with "closed"cells in which the conduction path for the electric current between apair of electrodes is confined to a fixed volume of liquid within thecompletely enclosed nonconductive structure to avoid externalinterference forces. However, such probe designs are substantiallyaffected by liquids carrying solids which deposited within the confinedareas of the probe. As the solids and other deleterious materials becomeentrapped in these complex probes, the calibration accuracy of theprobes begin to vary thereby affecting the conductivity measurement.Repeated standardizations and recalibrations or cleaning of the probeare required to maintain accurate measurements.

The present invention is a conductivity measurement probe of the closedcell type, but one which has a relatively simple design for confiningthe current flux within a channel and with such electrode configurationthat the calibration constant of the probe is substantially determinedonly by the physical dimensions of the flux channel and electrodespacing and not by the surface area, size, shape or corrosion andscaling of the electrodes.

SUMMARY OF THE INVENTION

In accordance with this invention, there is provided a conductivityprobe having a rigid body. A cylindrical flow passageway is formedwithin the body having electrically nonconductive wall members therebyproducing an insulated boundary defining a flux channel to polarizingcurrent. Inlet and outlet means pass aqueous liquid through the flowpassage. Four metallic electrodes of elongated configuration aredisposed in the flow passage and are positioned in parallel relationshipwith their longitudinal axis transverse to the flow passage. Theseelectrodes all reside in a plane containing a longitudinal axis of theflow passage and they are electrically isolated from the body. One pairof these electrodes are auxiliary electrodes for providing current flowin the flow passage. Another pair of these electrodes are referenceelectrodes which monitor polarization potential. The auxiliaryelectrodes may reside adjacent one another and the reference electrodesmay reside remote to one aanother at the limits of the flux channel andadjacent to each of the auxiliary electrodes. Preferably, each referenceelectrode is spaced equidistantly from the adjacent auxiliary electrode,and the auxiliary electrodes are spaced apart a greater distance fromone another than from the reference electrode. Electrically conductivemeans extend from the auxiliary and reference electrodes within thebody. The conductivity means are insulated from one another and from thebody so that the conductivity means form circuits through the electrodesduring conductivity measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical section (partial) through a first embodiment of theconductivity probe of this invention;

FIG. 2 is an end view of the conductivity probe shown in FIG. 1;

FIG. 3 is a vertical section (partial) through a second embodiment ofthe conductivity probe of this invention;

FIG. 4 is an end view of the conductivity probe of FIG. 3 taken alongline 3--3; and

FIG. 5 is an illustrative schematic circuit of the method of employingthe conductivity probe shown in the preceding figures according to thepresent invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In the drawings there are illustrated two embodiments of theconductivity probe of the present invention. Both of these conductivitymeasurement probes encompass all of the advantages of the presentinvention, but one may be more suitable for manufacture in one form orthe other depending on the particular circumstances of the user. InFIGS. 1 and 2 there is shown a first embodiment of a conductivitymeasurement probe 11 which is formed of a rectangular body 12 of a rigidmaterial. This material may be a polymerized plastic such as polyvinylchloride. Other materials may be employed for constructing the body 12,but preferably they are selected from electrically nonconductingplastics. The body 12 is provided with an axial opening 13 ofcylindrical configuration which extends through the body 12. Althoughthe surrounding portions of the body 12 may form electricallynonconducting wall members, it is preferred to insert a tubular member14 into the opening 13 to form a cylindrical flow passageway 16 throughthe body 12. The tubular member 14 is preferably formed of a compatiblematerial to the body 12 and polyvinyl chloride pipe of suitabledimensions is expecially suitable for this purpose. Where this materialis employed, an adhesive may be employed within the opening 13 so thatthe body 12 and tubular member 14 are integrally secured into a unitarystructure. As a result, the passageway 16 extends through the body 12and forms the flux channel to polarizing current which is to beestablished between electrodes contained therein. The tubular member 14has an inlet coupling 17 and an outlet coupling 18 by which connectionto an external source may be made so that the electrolyte or aqueousliquid may be passed through the passageway 16 in the directionindicated by the arrow 19. However, the inlet and outlet couplings maybe reversed so that the fluid may be passed through the passageway 16 inthe reverse direction. The probe 11 is symmetrical to flow of currentand therefore it is immune to the directional flow of liquid or currentaffecting conductivity measurements.

Four metallic electrodes 21, 22, 23, and 24 which have an elongatedconfiguration are disposed in the passageway 16. These electrodes maytake any suitable form, but preferably are substantially identicalcylindrical members. However, they need not be identical for purposes ofthe present invention. Identical cylindrical electrodes are preferredfor purposes of construction. For ease in construction, the electrodesare disposed symmetrically within the passageway 16. In addition, theelectrodes are positioned in parallel side-by-side relationship andreside with their longitudinal axis transverse to the passageway 16.Additionally, these electrodes reside in a plane containing thelongitudinal axis of the passageway 16. The electrodes may be mountedwithin the probe 11 by any suitable means, such as with an adhesive, butthey are electrically isolated from the body 12.

Although the electrodes are illustrated as traversing the wall membersof the tubular member 14, such arrangement is not necessary. Theelectrodes need only to be mounted so that they extend transversely intothe passageway 16. However, for purposes of convenient construction, thetubular member 14 is formed with aligned openings through its wallmembers to receive the electrodes. Each electrode carries an end member21a, 22a, 23a, and 24a, respectively, by which electrical conductors maybe secured to them. In order to accommodate such a relationship, thebody 12 is relieved to provide an opening into which internal pieces maybe received. This opening may be provided by a second axial opening 26formed coaxially with the opening 13. The axial opening 26 need notextend through the body 12, but may be terminated a short distance fromone end thereby forming an impermeable wall portion 27 adjacent theelectrode 21. Preferably, the electrodes are mounted within the tubularmember 14 in the mentioned openings prior to mounting the tubular member14 within the body 12. Also, it is desirable before this assembly thatthe electrical conductors 31, 32, 33, and 34 be secured to the endportions 21a, 22a, 23a, and 24a, respectively. After the tubular memer14 is inserted within the body 12 and secured thereto, these electricalconductors connect to an external electrical fitting 36 of any suitabletype such as a female multiterminal plug to receive an external shieldedand waterproof cable. The conductors are secured, such as by soldering,to terminal pins 37, 38, 39, and 41 of the connector 36. The connector36 is secured, preferably in watertight interconnection, to the body 12by screws 42.

After assembly of the electrical fitting 36 upon the body 12, theopening 26 is filled to a substantially void-free environment with anonconducting potting material. The potting material can be a polyvinylchloride compatible resin with suitable plasticizer forming a liquidmixture that can be poured into the opening 26 with the tubular memberupright having the outlet coupling 18 superimposed. With time passage,the liquid mixture sets into an impermeable, electrically nonconductingsolid material 44 substantially of the same rigidity as the body 12. Forexample, the potting material can be R-826 Liquid Resin from the RingChemical Company of Houston, Texas, and a setting catalyst such asVersamid 140 which is a polyamide castable plastic catalyst. The pottingmaterial hardens and unites the described elements into a unitaryconductivity measurement probe 11 which is substantially sealed to anyleakage of fluid about the electrodes through the wall members of thetubular member 14 into the body 12. If desired, the opening 13 may beenlarged slightly coextensively with opening 26 so that the pottingmaterial 44 completely surrounds the entire tubular member 14 where ittraverses the body 12. Thus, an opening 43 about the tubular member 14is also filled with the potting material 44.

The electrodes are positioned in a symmetrical arrangement within thebody 12. The electrodes 21 and 24, or 22 and 23, may be either referenceor auxiliary electrodes. However, best results are obtained withelectrodes 21 and 24 being references and electrodes 22 and 23 servingthe auxiliary function. In the probe 11, the electrodes 22 and 23 areauxiliary electrodes for providing current flow in the passageway 16.The electrodes 21 and 24 are reference electrodes for monitoring thepolarization potential therebetween which is induced by the current flowbetween the auxiliary electrodes 22 and 23. The reference electrodesshould be spaced at least three diameters from the adjacent auxiliaryelectrode so that the electrodes 21 and 24 are beyond the current pathsabout the auxiliary electrodes 22 and 23. As a result, the boundaryfilms about the reference electrodes 21 and 24 are not disrupted. Sincethe passageway 16 is circumferentially enclosed by insulating materialand current cannot pass beyond the outer electrode pair, this structuredelimits the electrodes into a well defined flux channel. Improvedresults are obtained with the auxiliary electrodes 22 and 23 spaced adistance greater than twice the spacing between any of the auxiliary andreference electrodes. With this arrangement, the reference electrodesare in the shadow of auxiliary electrodes beyond current flows. Thus,the electrodes 21 and 24 are "shielded" from stray currents. This effectis likened to a shadow about the reference electrodes where the lack oflight is lack of current flow from the auxiliary electrodes to thereference electrodes.

As has been mentioned, the construction, cross-sectional area, and otherphysical size parameters of the electrodes do not influence theoperability or the calibration of the probe 11 except in the secondorder relationship. Since the passageway 16 is cylindrical and theelectrodes are mounted therein in a symmetrical relationship, thecalibration constant for the conductivity measurement probe 11 issubstantially determined only in the first order by the spacing betweenthe electrodes 22 and 23 relative to the cross-sectional area (in atransverse plane) of the passageway 16. The effect of electrode size(diameter and length), corrosion attack and incomplete scale coverage ofelectrodes are only third order effects. Thus, corrosion, pitting andother physical changes in the electrodes do not affect the calibrationconstant for the probe 11. Should the mentioned ratios concerning theflux channel and inner electrode pair spacings within the passageway 16be changed, then and only then, will the calibration constant change.Since the possibility of changing the length of the flux channel withinthe passageway 16 during use is relatively impossible, the probe 11 isof the "closed" cell type so that external influences about the probe 11do not interfere with the conductivity measurement being undertaken.Also, any change of the cross-sectional area of the flow passageway 16can only occur by deposition of solids therein. Usually, such depositionis of uniform dimension throughout the circumference and length of thepassageway 16. Thus, the influence upon the cross-sectional ratio factordetermining the conductivity probe's cell constant is very minute. Thus,the probe 11 maintains a cell constant for most practically anyenvironmental use until the electrodes are completely destroyed bycorrosion or other physical reduction to an inoperative size, namely,disappearance.

The determination of the calibration constant for the corrosionmeasurement probe of the present invention is determined by uniquerelationship of mathematical definition for the probe employed therein.More particularly, these probes are defined by the followingrelationship: i = v(A/d)1/e wherein i is the current between theauxiliary electrodes, v is the incremental change in polarizationpotential between the reference electrodes, e is the resistivity of theelectrolyte (e.g., water), A is the cross-sectional area of thepassageway 16, and d is the distance between the auxiliary electrodes 22and 23. Since A and d are fixed in relationship, then i = v(K/e) whereinK is the "cell" constant. Rewriting the equation for the system i/v =K/e = KΔ wherein Δ is the conductivity of the electrolyte. Since i and vare readily measured, the conductivity is easily found when v is nearzero, i = K'Δ since K' is the system calibration constant pursuant toOhm's Law.

The conductivity measurement probe 11 can be manufactured to an exactlyprecise calibration constant. However, the expense of such technique ofmanufacture need not be suffered. For this purpose, the probe 11 isprovided with a very unique method of adjusting the system calibrationconstant K' to a particular desired value and especially to acalibration with a particular external associated electrical system. Forexample, in the equation i = K'Δ, a one micorampere can be equivalent toa conductivity of 1,000 micromhos where K' = 0.001 (approximately). Forthis purpose, the probe 11 is constructed with a system calibrationconstant K which is smaller than the desired value. Then, the probe 11is connected through the electrical fitting 36 to an external electricalsystem. Current is passed through a known conductivity liquid betweenthe electrodes 22 and 23 and the incremental change in polarizationpotential is measured through the electrodes 21 and 24. By therelatively simple earlier described calculation, the system calibrationconstant K' for the probe 11 can be determined.

For the purpose of determining K' and conductivity measurements, thecircuit shown in FIG. 5 can be employed. In particular, thecharacteristics of the probe 11 are shown schematically in FIG. 5 andlike parts have like reference numerals. A suitable current source, suchas a battery 46, is placed in series with an ammeter 47 (e.g., a 0-500micrometer) and a rheostat 48. By this means, a current flow isestablished between the electrode 22 and 23 with a suitable knownelectrolyte in the passageway 16. The electrolyte may be contained in astatic environment or in a flowing condition. The particular flow stateis immaterial to the operation of the probe 11. For example, theelectrolyte may be distilled and gas-free water which has a knownconductivity. The potential induced between the electrodes 21 and 24 ismeasured by a suitable high impedance voltmeter 49 (e.g., a 0-50millivoltmeter). The instrumentation described in U.S. Pat. No.3,717,566 may be employed for this purpose to good advantage. With thisequipment, sufficient current is passed between the electrodes 22 and 23so that there is about a 10 millivolt polarization potential changebetween the electrodes 21 and 24.

Should the cell constant K' be within the desired range with thisparticular instrumentation, the characteristics of the flux channelwithin the probe 11 can be adjusted for any specific calibration cellconstant. For this purpose, an insulating member is mounted within thebody 12 so that it may move transversely within the passageway 16 at alocation substantially equidistantly from the auxiliary electrodes 22and 23. Although the insulating member may be of any suitable form, itis preferred that the insulating member is threadedly mounted so thatprecise transverse movement within the passageway 16 may be obtained.For this purpose, the insulating member may be a screw 40 constructed ofplastic material such as Nylon or polyvinyl chloride. The screw 40 isthreaded through the body 12, the wall member of tubular member 14, andany intervening potting material 44. Adjustment of the screw 40 movesits end 40a transversely within the passageway 16. The movement of thescrew between positions influences the cross-sectional area of the fluxchannel to current passages between the electrodes 22 and 23. Bydeflection of the lines of force which such current follows, theconductivity cell constant of the probe 11 can be adjusted to anyprecise value within a reasonable range. Once the screw 40 is adjusted,the probe 11 will remain within calibration. If desired, the screw 40 isreceived within a cylindrical cavity 45 in the body 12. The remote endof the cavity 45 carries an internal threaded portion 45a in which athreaded plastic plug 50 is received. As a result, fluids cannot movebetween the exterior environment into contact with the screw 40, nor topossibly enter the passageway 16. As a result, the probe 11 issubstantially secured against the influences of external material whichmight influence the conductivity measurement.

Referring now to FIGS. 3 and 4, there is shown another embodiment of thepresent conductivity measurement probe which employs a flow passagewayincluding electrodes forming a bounded flux channel so that the systemconstant K' of the probe is first order independent of the ratio of thelength of the flux channel, electrode dimensions, etc. However, thisembodiment has construction features which are desirable where the probeis to be inserted directly into an existing polyvinyl chloride pipingsystem carrying the electrolyte. The conductivity measurement probe 51is formed of a rigid body 52 which may be of any suitable material.Preferably the body 52 is formed of polyvinyl chloride plastic material.The body 52 may be rectangular in configuration and carries acylindrical opening 53. The opening 53 has electrically nonconductingwall members which may be provided by the body 52 when constructed of arigid plastic, nonconducting material. The cylindrical opening 53 formsa cylindrical passageway 56 which extends through the body 52. A pair ofpipe segments 57 and 58 of the existing piping system serves as inletsand outlets for passing aqueous liquid through the passageway 56. Thepipe segments are secured within the body 52 by an adhesive or othermeans. If desired, the functions of the pipe segments 57 and 58 may bereversed. Since the probe 51 is symmetrical relative to the conductivitymeasuring portions, fluid can be introduced into the passageway 56 inthe direction indicated by the arrow 59 with equal facility and resultsas with flow in the reverse direction.

Four elongated metallic electrodes 61, 62, 63, and 64 are disposedtransversely within the passageway 56 and positioned in a parallelrelationship with their longitudinal axis. These electrodes reside in aplane containing the longitudinal axis of the cylindrical passageway 56and are electrically isolated from the body 51. The dimensions and othercriteria of the electrodes can be the same as has been described forprobe 11 in the preceding embodiment.

In order to facilitate construction of the probe 51, the body is made inupper and lower halves joined at meeting surface 67 with a polyvinylchloride cement. The body 52 has a rectangular opening 66 which permitsaligned holes to be prepared in the opposite sidewall surfacessurrounding the passageway 56 within the body 52. The electrodes arereceived in fluid-tight engagement within these holes. The electrodesmay extend completely through the passageway 56, or only partiallytherethrough, without detracting from the advantageous characteristicsof the probe 51. The electrodes carry at their upper extremities (asillustrated in FIG. 3), end parts 61a, 62a, 63a, and 64a to which aresecured electrical conductors 71, 72, 73, and 74. These electricalconductors extend from the electrodes to an external electrical fitting76 carried on the exterior of the body 52.

The electrical fitting 76 may be of any conventional type such as afemale screwed cap connector for a waterproof electrical cableattachment to the probe 51. In this regard, the electrical fitting 76carries terminal pins 77, 78, 79, and 81 to which the electricalconductors 71, 72, 73, and 74, respectively, are connected. Theelectrical fitting 76 can be secured to the body 52 by screws 82 orother suitable fasteners. After the electrical conductors are securedbetween the electrodes and the terminal pins of the electrical fitting76, the opening 66 is filled with any suitable potting compound 84. Thiscompound may be of the same type of plastic material and catalystdescribed for the previous embodiment. End molding pieces (not shown)may close the sides of opening 66 so that the liquid potting compoundfills the body 52 to substantially void-free completeness. After thepotting compound has set to a state of rigidity, the exposed parts ofthe electrodes, the electrical conductors and the terminal pins ofelectrical fitting 76 are contained in a fluid-tight environment so thatno fluids pass between the passageway 56 and the external portions ofthe body 52.

Although the probe 52 could be constructed to an exact conductivity cellconstant K' (e.g., 1 milliampere = 1,000 micromhos), it may bedesirable, as in the preceding embodiment to provide an insulatingmember which can transversely be moved within the passageway 56. Forthis purpose, an insulating member, such as a plastic screw 86, ispositioned for transverse movement into the passageway 56 at an equalspacing from the electrodes 62 and 63. The screw 86 is adjusted bymoving its end 87 within the passageway 56 until the system constant K'of the probe 51 is at the conductivity of a standard electrolytesolution which fills the passageway 56. The screw 86 is protectedagainst accidental movement or injury by a plastic nipple 88 carriedwithin a recess 89 formed in the body 52. The nipple 88 is securedwithin the recess 89 by an adhesive such as polyvinyl chloride cement.The nipple 88 carries internal threads 91 to receive a plastic plug 92.

The characteristics of the electrodes and their spacing within the probe51 is identical to that described third the probe 11 in the precedingunit embodiments. The electrodes 62 and 63 are auxiliary electrodes forpassing current through the electrolyte in the passageway 56. Thereference electrodes 61 and 64 are placed at a position without thepolarization currents. Therefore, the electrodes 61 and 64 and thepassageway 56 define the extremities of the flux channel. Theelelctrodes 61 and 62, and 63 and 64, should be at an equal spacing,whereas the electrodes 62 and 63 should be at a spacing greater thanbetween the adjacent reference and auxiliary electrodes. Preferably,spacing between the electrodes 62 and 63 is twice that between theauxiliary and reference electrodes.

The probe 51 is employed in the same manner as the probe 11 of FIG. 1.The probe 51 connects to a suitable external circuit, such asschematically illustrated in FIG. 5, and current is passed between theelectrodes 62 and 63 causing a predetermined shift in polarizingpotential between the electrodes 61 and 64. The previously mentionedformula is employed for detemining the conductivity of the electrolytecontained within the passageway 56. The passageway 56 may be filled witha known electrolyte having a known conductivity. The response of theprobe 51 is then determined, the screw 86 adjusted until the desiredcell calibration constant is obtained, and plug 92 reinstalled. Thus,the probe 51 can be brought to any desired calibration relative to theammeter 47 illustrated in FIG. 5.

From the foreging there has been described two embodiments of a probe ofthe present invention in which the conductivity is a function only ofthe distance between the auxiliary electrodes and physical dimensions ofthe flux channel provided therein. The dimensions, corrosion attack,pitting, fouling of the electrodes, including their shapes, size andsurface areas have only a third order effect upon the measurement ofconductivity. The probes have all the advantages of "closed" cell d.c.measurement systems of conductivity and are not influenced by thedeposition of scale other than would severely change the dimension ofthe flux channel.

Various modifications and alterations in the described probe will beapparent to those skilled in the art from the foregoing descriptionwhich do not depart from the spirit of the invention. For this reason,these changes in structure are desired to be included within the scopeof the present invention. The appended claims define the presentinvention; the foregoing description is to be employed for setting forththe present embodiments as illustrative in nature.

What is claimed is:
 1. A conductivity probe comprising:a. a body formedof a rigid insulating material and having a cylindrical openingtherethrough; b. a tubular electrically nonconducting member positionedcoaxially within said opening of said body with an annulus therebetweenextending at least partially through said body and said tubular memberforming a cylindrical flow passage within said body thereby providing aninsulated boundary defining a flux channel to polarization current; c.said tubular member extending beyond the exterior surface of said bodyand carrying end connections adapted to function as inlet and outletmeans for passing aqueous liquid through said flow passage; d. fourmetallic electrodes of elongated configuration disposed in the flowpassage and mounted with at least one of their ends penetrating the wallof said tubular member within said annulus between said body and saidtubular member, and said electrodes positioned in parallel relationshipwith their longitudinal axes transverse to said flow passage andresiding in a plane containing the longitudinal axis of said flowpassage; e. one pair of said electrodes being auxiliary electrodes forproviding current flow in said flow passage and another pair of saidelectrodes being reference electrodes serving as polarization potentialmonitors, said auxiliary electrodes residing adjacent one another andsaid reference electrodes residing remote from one another at the limitsof said flux channel and adjacent each auxiliary electrode; f. each ofsaid reference electrodes spaced equidistantly from the adjacentauxiliary electrode, and said auxiliary electrodes spaced apart agreater distance from one another than said auxiliary electrodes fromsaid reference electrodes; g. an elongated opening provided in said bodyintersecting said annulus and said elongated opening disposed injuxtaposition with said tubular member with one end of said electrodesbeing exposed by said elongated opening, and said elongated openingcommunicating with the exterior of said body; h. electrically conductivemeans extending from said auxiliary and reference electrodes within saidelongated opening to the exterior of said body; and i. a cast insulatingmaterial filling void-free said annulus and elongated opening forintegrally securing said tubular member, electrodes, electricallyconductive means and body into a fluid-tight and rigid relationship, andsaid cast insulating material insulating said conductive means from eachother in said body whereby said conductive means form circuits throughsaid electrodes during conductivity measurements.
 2. The conductivityprobe of claim 1 wherein said electrodes are substantially identicalferrous members and the spacing between said auxiliary electrodes isgreater than twice the spacing between one of the auxiliary electrodesand the reference electrode adjacent thereto.
 3. The conductivity probeof claim 1 wherein said body is provided with a transverse openingintersecting said flow passage, and an insulating member is mounted insaid transverse opening for movement within said flow passage betweensaid auxiliary electrodes to position effecting the apparentconductivity constant for said probe provided by the cross-sectionalarea of said flux channel.
 4. The conductivity probe of claim 3 whereinsaid insulating member is threadedly mounted in said transverse openingin said body for precise movement within said flow passage.
 5. Theconductivity probe of claim 3 wherein said insulating member is mountedin said transverse opening for movement transversely in said flowpassage and substantially equidistant from said auxiliary electrodes. 6.A conductivity probe comprising:a. a body formed of a rigid insulatingmaterial and divided into two parts with a planar meeting surface, andsaid body having a cylindrical opening formed therein with thelongitudinal axis of said opening residing along said planar meetingsurface; and said cylindrical opening providing an insulating boundarydefining a flux channel; b. adhesive means securing the parts of saidbody of said planar meeting surface into a leakproof interconnection; c.a pair of tubular members secured in fluid-tightness within said bodyand aligned coaxially of said cylindrical opening, said tubular membersextending beyond said body and carrying end connections adapted to serveas inlet and outlet means for passing aqueous liquid through said flowpassage; d. an elongated opening provided in said body and saidelongated opening extending in said body at least partially the lengthof said cylindrical opening but separated therefrom by an imperforatewall of rigid insulating material, and said elongated openingcommunicating with the exterior of said body. e. four metallicelectrodes of elongated configuration disposed in the flow passage andmounted through said imperforate wall of said insulating material withat least one of their ends extending into said elongated opening, andsaid electrodes positioned in parallel relationship with theirlongitudinal axes transverse to said flow passage and residing in aplane containing the longitudinal axis of said flow passage; f. one pairof said electrodes being auxiliary electrodes for providing current flowin said flow passage and another pair of said electrodes being referenceelectrodes serving as polarization potential monitors, said auxiliaryelectrodes residing adjacent one another and said reference electrodesresiding remote from one another at the limits of said flux channel andadjacent each auxiliary electrode; g. each of said reference electrodesspaced equidistant from the adjacent auxiliary electrode, and saidauxiliary electrodes spaced apart a greater distance from one anotherthan from said reference electrodes; h. electrically conductive meansextending from said auxiliary and reference electrodes within saidelongated opening to the exterior of said body; and i. a cast insulatingmaterial filling said elongated opening in said body substantiallyvoid-free for integrally securing said electrodes, electricallyconductive means and body into a fluid-tight and rigid relationship, andsaid cast insulating material insulating said conductive means from eachother in said body whereby said conductive means form circuits throughsaid electrodes during conductivity measurements.
 7. The conductivityprobe of claim 6 wherein said electrodes are substantially identicalferrous members and the spacing between said auxiliary electrodes isgreater than twice the spacing between one of the auxiliary electrodesand the reference electrode adjacent thereto.
 8. The conductivity probeof claim 6 wherein said body is provided with a transverse openingintersecting said flow passage and an insulating member is mounted insaid transverse opening for movement within said flow passage betweensaid auxiliary electrodes to position effecting the apparentconductivity constant for said probe provided by the cross-sectionalarea of said flux channel.
 9. The conductivity probe of claim 8 whereinsaid insulating member is threadedly mounted in said transverse openingin said body for precise movement within said flow passage.
 10. Theconductivity probe of claim 8 wherein said insulating member is mountedin said transverse opening for movement transversely in said flowpassage and substantially equidistant from said auxiliary electrodes.